In most cases, grating structures (such as Bragg gratings) are formed in an optical fiber core by irradiating the fiber from the side (i.e., in a direction transverse to the length of the fiber) with a UV beam (i.e., actinic radiation) that introduces a patterned change in the refractive index of the core region. The amount of change in refractive index that is created is defined as the “strength” of the grating, where a large change in refractive index (i.e., large Δn) is defined as a “strong” grating. As is well known, the periodicity of this change is used to control the properties of the grating, including the wavelength(s) filtered or passed by the structure.
When attempting to form gratings in multicore fibers, undesirable variations in the “strength” of the grating formed in each separate core region will occur, as a result of the lensing properties of the fiber itself. FIG. 1 illustrates this principle. A side view of an exemplary multicore fiber 1 is shown, where multicore fiber 1 includes a first core 2 in essentially the center region of the fiber and a second, offset core 3. In this particular orientation, offset core 3 is shown as being offset to the left of first core 2 along the z-axis direction.
A beam 4, used to create grating structures within both first core 2 and offset core 3, is shown as illuminating multicore fiber 1 from the left-hand side. Beam 4 may be defined as a writing beam ray. As a result of the curved surface S of multicore fiber 1, beam 4 will converge as it passes through the diameter D of multicore fiber 1. Therefore, multicore fiber 1 can be said to exhibit the property of a cylindrical lens, causing beam 4 to focus during its propagation across the width of the fiber. In this particular example, beam 4 ultimately focuses at a point F beyond multicore fiber 1.
As a result of the beam focusing, the intensity of the beam increases as it passes through the width of the fiber. While not a problem with standard single-core fibers, this variation in beam intensity as a function of position across the fiber diameter is problematic to the process of inscribing gratings in multicore fibers. In particular, the increase in intensity will naturally increase the strength of the grating formed within the various core regions, as the beam propagates from the left to the right. For the arrangement as shown in FIG. 1, therefore, the strength of a grating formed in offset core 3 will be less than the strength of a grating formed in first core 2. First core 2 is identified as having a grating with a refractive index difference Δna and offset core 3 is identified as having a grating with a refractive index difference Δnb, where Δna>Δnb. Non-uniformity in grating strength is typically an undesirable result.
Moreover, if a multicore fiber becomes twisted during grating inscription, a given core will experience varying intensity along its length. Therefore, the grating strength of a structure formed along a length of a twisted core will also vary longitudinally along the length of the fiber. In at least the circumstance where the multicore fiber gratings are used as a shape sensor, variations in grating strength along a given core will require larger dynamic range in any interrogator that processes the light scattered from this variable-strength grating.
For these reasons and others, it is desirable to reduce variations in grating strength present in multicore fibers.